Magnetic resonance imaging (MRI) is a non-invasive imaging technique that provides images with excellent soft tissue contrast. MR imaging contrast of biological tissues is generally based on the relaxation properties of water protons, which usually reflects a combination of spin-spin (T2) and spin-lattice (T1) relaxation. MRI detects the signal from bulk protons in biological tissues as they have long T2 relaxation.
Chemical Exchange Saturation Transfer (CEST) is a technique that provides an indirect way of detecting the signal from exchangeable protons with bulk water. Those exchangeable protons are typically associated with macromolecules or small metabolite molecules. CEST imaging uses an off-resonance saturation pulse at the resonance frequency of exchanging protons to null the signal from exchangeable protons in order to indirectly decrease bulk water signal through chemical exchange, creating a detectable contrast from bulk water. CEST contrast can be turned “on” and “off” by simply changing the saturation RF frequency and is highly specific to CEST agents that possess distinct resonance frequencies. FIG. 1 illustrates a pulse diagram of CEST imaging. Here the readout sequence is shown as a single pulse acquire method, but one could use any type of imaging readout to measure the CEST effect.
By way of brief overview of CEST in biological tissues, consider two nuclear spin I=1/2 systems, A (solvent) and B (solute), with a distinct chemical shift difference, Δω, and an exchangeable proton on the solute that exchanges with water solvent protons. In a static magnetic field, application of a long low power RF pulse at the resonance of (B) without affecting the resonance of A leads to the equalization of the populations in the two spin states of B, a situation referred to as spin saturation, and no signal is observed from spin B. Since the B spins are in exchange with that of A spins, the saturated magnetization is transferred to A spins and a concomitant decrease in the signal intensity of the A spins occurs. Subsequently, longitudinal relaxation returns each nuclear spin system to its equilibrium values and eventually the system reaches a steady state. This effect is termed chemical exchange saturation transfer (CEST). The saturation transfer magnetization is then imaged to detect the CEST effect from nuclear spins B. In order for the CEST effect to be efficiently observed, the slow to intermediate exchange condition (Δω>k) must be fulfilled.
In implementing this method in vivo, and interpreting the results there are several issues one has to address: (i) the direct saturation of water and the background magnetization transfer effect in biological tissues; (ii) the amplitude and duration of the saturation pulse and efficiency of saturation. To account for the first, one has to obtain two images, one with saturation at the resonance frequency of exchanging spin(s) and the other at the equal frequency offset difference on the other side of the bulk water peak and then compute the difference as CEST asymmetry (CESTasym) ratio, which provides the CEST effect of the source spins. This is given by Equation [1],
                                          CEST            asym                    ⁡                      (                          Δ              ⁢                                                          ⁢              ω                        )                          =                                                            M                sat                            ⁡                              (                                  -                  Δω                                )                                      -                                          M                sat                            ⁡                              (                Δω                )                                                          M            0                                              [        1        ]            where M0 is the equilibrium magnetization when saturation frequency is set far from the water resonance, such as 20 ppm down field of the water resonance, Msat (±Δω) are the magnetizations obtained with saturation at a ‘+’ or ‘−’ Δω offset of the water resonance. The role of amplitude and duration of the saturation pulse can be incorporated into a general solution of a two-site exchange model in the presence of RF saturation as shown in Equation [2]:
                                          CEST            asym                    ⁡                      (            Δω            )                          =                                            k              ⁢                                                          ⁢              α              ⁢                                                          ⁢              f                                                      R                                  1                  ⁢                  w                                            +              kf                                ⁡                      [                          1              -                              ⅇ                                                      -                                          (                                                                        R                                                      1                            ⁢                                                                                                                  ⁢                            w                                                                          +                        kf                                            )                                                        ⁢                                      t                    sat                                                                        ]                                              [        2        ]            where k is exchange rate (s−1 or Hz), α is the factor that accounts for suboptimal saturation with 1 describing complete saturation (at high enough B1 amplitude of saturation pulse). For MI CEST imaging, f=n[MI]/2[H2O] is the fraction of exchangeable protons on MI (n=6 for six —OH), R1w (=1/T1w) is the longitudinal relaxation rate of water protons and tsat is the length of the of the saturation pulse. MI CESTasym contrast is referred to as MICEST.
CEST contrast can originate from endogenous amide, amine, and hydroxyl protons or from exchangeable sites on exogenous contrast agents. The CEST effect from amide protons of mobile proteins/peptides has been reported and the potential to measure the pH using these amide protons in biological conditions also has been explored. However, the CEST effect from amide protons is not protein/peptide specific and thus cannot be used in detection of a particular protein or peptide. Thus, disease specific CEST biomarkers which have exchangeable protons at distinct resonance frequencies away from water resonance will have great clinical and commercial significance.
CEST has been used to study different metabolites. For example, labile amide proton (—NH) exchange with bulk water has been exploited to map pH changes in tissues as well as the protein content in the brain, while —OH exchange has been used to measure the proteoglycan concentration in cartilage as well as glycogen concentration changes in the liver. CEST has also been used to generate exogenous contrast using contrast agents with exchangeable protons. Applying RF irradiation at the exchangeable proton resonance leads to reduction in the bulk water signal that can be detected as a negative contrast with proton MRI.
Although 1H magnetic resonance spectroscopy (MRS) techniques have been used to measure metabolites, these approaches are riddled with technical difficulties due to several confounding factors, such as very low spatial resolution, high detection limits, spectrum overlapping with other metabolites and complicated unreliable signal processing. It is desired to provide high spatial resolution, high specificity detection of various metabolites in the body without overlapping with other metabolites and to turn on and off the CEST contrast. Such methods are desired to assist in the diagnosis and treatment monitoring of various disorders in the brain (e.g. multiple sclerosis (MS), Parkinson's disease, stroke, Alzheimer's disease, etc.) and liver (e.g. cirrhosis and carcinoma), for example.